NOTE 1 – This quantity is computed as follows: if N_NS is the number of customers affected and the supply of customer j is interrupted n_j times during a time interval of duration T, then

${\displaystyle f_{\text{CI}}=\frac{{\sum }_j{n}_j}{N_{\text{NS}}T}}$

When determining N_NS, each customer affected must be counted only once, regardless of the number of interruptions experienced during that time interval.

This quantity can be used to describe past performance or to estimate expected future performance.

NOTE 2 – Usually, only sustained interruptions are considered.

NOTE 3 – The term “customer” represents the ultimate customer and not some intermediate distribution system or company.